The Synchronous Digital Hierarchy (SDH) and its North-American equivalent, the Synchronous Optical Network (SONET), are the globally accepted, closely related and compatible standards for data transmission in the public wide area network (WAN) domain. Recently, SDW/SONET has also been adopted by the ATM Forum as a recommended physical-layer transmission technology for ATM (Asynchronous Transfer Mode) network interfaces.
SONET and SDH govern interface parameters; rates, formats and multiplexing methods; operations, administration, maintenance and provisioning for high-speed signal transmission. SONET is primarily a set of North American standards with a fundamental transport rate beginning at approximately 52 Mb/s (i.e., 51.84 Mb/s), while SDH, principally used in Europe and Asia, defines a basic rate near 155 Mb/s (to be precise, 51.84×3=155.52 Mb/s). From a transmission perspective, together they provide an international basis for supporting both existing and new services in the developed and developing countries.
For transmitting data, SDH and SONET use frame formats transmitted every 125 μs (8000 frames/s). Because of compatibility between SDH and SONET, their basic frames are similarly structured, but differ in dimension, which fact reflects the basic transmission rates of 155.52 and 51.84 Mb/s, respectively. To be more specific, a basic frame format of SDH is 9 rows of 270 byte columns, or 2430 bytes/frame, corresponding to an aggregate frame rate of 155.52 Mb/s. For SDH systems, the mentioned basic frame transmitted at the rate 155.52 Mb/s forms the fundamental building block called Synchronous Transport Module Level-1. For SONET systems, the basic frame has dimensions of 9 rows by 90 bytes (270:3) and, being transmitted at the rate 51.84 Mb/s (155.52:3), forms the appropriate fundamental building block called Synchronous Transport Signal Level-1 (STS-1).
Both the SDH, and the SONET systems are based on the hierarchical principle of composing higher order signals (so-called high order virtual containers) from lower order signals (so-called lower order virtual containers). For example, the STM-1 signal, according to SDH mapping scheme, contains a signal called AU-4 that, in turn, carries a signal VC-4. The virtual container VC-4 can be mapped from a number of lower order signals. In SONET system, the STS-1 signal contains a signal AU-3 that in turn carries a signal VC-3. Similarly, the VC-3 can be composed from several lower order signals.
SDH also includes signals of Synchronous Transport Level 4, 16 and 64 (so-called VC4-N) which constitute 4, 16 or 64 independent VC-4 signals. An analogous arrangement exists in SONET (signals STS-3, STS-12, STS-48 etc.)
SDH and SONET are known to support data streams having rates higher than the fundamental building block If there are services requiring a capacity greater than 155 Mbps, one needs a vehicle to transport the payloads of these services. In SDH, so-called concatenated signals, for example VC4-Nc, are designed for this purpose. STM-4 signal having a data rate 622.08 Mb/s (4×155.52 Mb/s) is one of the high order signals in the SDH system. Payload of the STM-4 signal is generated by byte-interleavingly multiplexing four payloads of STM-1 (or four AU4, or four VC4) signals. Concatenated VC4 (VC4-Nc) is characterized by a common synchronous payload envelope being the N-fold VC4 signal, and by a common column of service bytes called POH (Path Overhead); for transmitting, such a signal needs a number of adjacent time-slots.
Operation of rearrangement is known in SDH/SONET signals transmission.
For transmitting a number of SDH signals, say, 10 independent VC4 containers via a telecommunication link such as an optic link, a well known TDM (Time Division Multiplexing) principle is used.
According to this principle, a byte-interleaving multiplexer intermittently transmits bytes of the 10 containers via an optic link in a manner that specific time slots are assigned to bytes of the respective specific containers. Let the optic link allow for transmitting bytes in 16 timeslots, with a frequency 2.5 GHz, which is sufficient for a high rate SDH signal STM16. For example, the initial arrangement at the transmitter side is such that bytes of VC4 containers Nos. 1 to 5 are sent in respective time slots 1 to 5, and bytes of VC4 Nos. 6 to 10 are transmitted in time slots 9 to 13.
Suppose, that a new signal should be transmitted via the same optic link, and the bandwidth of the link would theoretically allow it (i.e., there are vacant time slots). However, a simple sum of the vacant time slots might be insufficient for transmitting the new signal if it requires several adjacent (sequential) slots. For example, a concatenated signal VC4-4c requires 4 adjacent time slots for its transmission, and in our example we don't have such slots available. It would therefore be useful to regroup the transmitted 10 separate VC4s so as to free one window of four consequent time slots for transmitting the new, concatenated signal.
In another example, two AU4 virtual SDH containers are transmitted via a link, and neither of them is “fully packed”: each AU4 signal contains 30 lower order signals (containers) TU12. It should be noted that according to the SDH hierarchy, 63 TU12 signals might be mapped in one AU4 container. Could all the TU12s be rearranged into one of the AU4 containers, the second AU4 container would be vacant for transmitting an additional signal, for example a new VC4 signal that requires almost the whole AU4 capacity. (One AU4 container comprises one VC4 container and an additional 9-byte row of so-called Administrative Unit pointers that serve, inter alia, for allocating the beginning of a VC4 payload in the frame of the transmitted signal).
It should also be emphasized that the rearrangement, if needed, is to be provided while the traffic proceeds i.e., without affecting it.
Some technologies of rearrangement are described in the prior art, and all of them relate to complex procedures to be performed inside a so-called cross-connect network element.
For example, U.S. Pat. No. 5,987,027 to Alcatel describes a connection procedure for finding by rearrangement a path for multirate, multicast traffic through an SDH cross-connect. If no free path for a new payload through the SDH switching hardware is available, the switching procedure looks for a path that is adequate and blocked by the least existing payload capacity. Connections for existing payloads that must be moved to make way for the new payload are queued and the connection procedure is applied recursively, to each in turn, until the queue is empty.
U.S. Pat. No. 5,408,231 to Alcatel Network Systems relates to a method and system for finding a path through a communication matrix (forming part of a cross-connect network element), preferably in a rearrangeable matrix. The method performs a so-called process of pumping the input stage array, output stage array and center stage array of the matrix using information on the idle input link array and the idle output link array to determine an optimal center stage switch.
U.S. Pat. No. 5,343,194 to Alcatel Network Systems also discloses a method to immediately connect and reswitch connection configurations through a rearrangeable communications matrix, using an optimization procedure that targets the minimal possible rearrangements.
U.S. Pat. No. 5,345,441 to AT&T Bell Laboratories describes a procedure of hierarchical path hunt for establishing a switched connection of a given bandwidth as a collection of a plurality of connections of smaller bandwidths of different sizes. The path hunt uses a hierarchy of status tables, corresponding to the hierarchy of rates, for each time switching element in the network. To maximize the path-hunt efficiency while maintaining non-blocking performance, the path-hunt follows a search hierarchy for lower-rate connections that first searches for matching partially full time-slot entries in higher rate status tables, and uses idle time-slot entries in higher-rate status tables only as a last resort.
U.S. Pat. No. 4,417,244 to IBM corp. discloses yet another method for rearranging a three stage (primary, intermediate, tertiary) switching network to permit data to be transmitted from any primary outlet to any given tertiary inlet. Two call rearranging buses are provided to assure that each signal path being rearranged is maintained to prevent data transmission dropout. Primary to intermediate and intermediate to tertiary paths are rearranged one at a time using the call rearranging buses to move free primary and tertiary links to a single intermediate matrix. It should be noted that, for rearrangement, some existing connections are to be broken and then made again in a queue.
U.S. Pat. No. 5,482,469 relates to a dual monitor self-contained six port digital signal cross-connect module. There is described an internal arrangement of a housing with a compact, self-contained, six jack port, dual monitor, digital signal cross-connect switching module. A first monitor jack port and a second monitor jack port are mounted in the housing, each being adapted to receive an electrical plug. A plurality of modules comprise a system having provisions for cross-connect switching, rerouting, repair, patch and roll and monitoring. The six jack port digital switching module paired with a like unit has an input jack port, an output jack port, a cross-connect input jack port, a cross-connect output jack port, and four multi-purpose monitor jack ports. Each makes a make before brake switch providing without a loss of signal, the means for bridging, disengaging, isolating, connecting respective conductors and terminating input and output signals when an electric plug is inserted into a suitable jack port. Though U.S. Pat. No. 5,482,469 is declared as intended for monitoring, testing, maintenance, installation and the like of electrical signal transmission systems, its description is focussed on internal assemblage of the housing and does not address the procedure of performing the connections. It therefore does not provide information to judge whether the re-connection is really provided without any loss of signal.
U.S. Pat. No. 6,018,576 relates to a method and an apparatus for automated node-based normalization after restoration of a network. After a failure in the network is repaired and a specified time period is passed, the end nodes perform a sequence of tasks to execute a modified form of a path-and-roll normalization. The process of switching from the restoral route to the original fixed route is performed under the patch-and-roll method, according to which each end node transmits traffic over both a restoral route and the original traffic route that has been fixed. Each end node confirms receipt of signals over the fixed traffic route. Thereafter, each end node switches to receiving live traffic from the restoral route to the fixed traffic route and stops transmitting over the restoral traffic route. According to U.S. Pat. No. 6,018,576, the end nodes finally instruct the other nodes along the restoral route to disconnect the restoral route. The confirmation message ensures that both of the end nodes receive the live traffic over the original, fixed traffic route so that at no time is traffic disrupted in the network. However, U.S. Pat. No. 6,018,576 neither describes nor suggests how the goal of non-disruption of the live traffic in the network can really be achieved.
It is therefore the situation that so far no errorless on-line rearrangement and switching procedure is described in the art. Usually in practice, an NDF alarm (New Data Flag) accompanies any rearrangement process in SONET/SDH. This alarm manifests the presence of a so-called frame slip which becomes sensible in a period of approximately three standard frames after the switching is done, and indicates that the rearranged data streams are “seamed” defectively.